MartenTrendelenburgStv.Chefarzt/KlinikInnereMedizin

STURZABKLÄRUNG

77jährigerPatient- Anamnese

¨ PatientkommtvonzuHause.¨ 17:30vonEhefrauaufFussboden vorgefunden(Sturzereigniss unklar).Patientberichtete,erhabezuHausebeihäuslichenTätigkeitendasGleichgewichtverlorenundseirückwärtsauf‘s KreuzunddenRückengefallen.KeinSchwindel,keineBewustlosigkeit.

¨ AllgemeinzustandsverschlechterungundKraftlosigkeitseiteinpaarTagen.

¨ HustenmitAuswurfseit2Tagen,amEintrittstagangeblichFieber.

¨ HäufigesWasserlösenmitHarndrangseit2Monaten.

77jährigerPatient- Vorgeschichte

¨ LeichtesdementiellesSyndrom¤ 1x/Woche(Eintrittstag)inTagesstättedesFelix-Platter-Spitals

¨ UnklareGangstörung¤ keinHinweisaufeineneurodegenerativeErkrankungimPET/CT10Monatevorher(AbklärungviaNeurologie)

¨ SubakuterNSTEMIbeikoronarer2-AstErkrankung2009

¨ ArteriellerHypertonus¨ St.n.Subarachnoidalblutung undKontusionsblutungennachSturzunklarerUrsache2003

77jährigerPatient- Medikamente

¨ Pantoprazol 40mgTbl.1-0-0¨ Amlodipin 5mgTbl.1-0-0¨ Simvastatin40mgTbl.0-0-1/2¨ Aspirincardio 100mgTbl.1-0-0¨ VitaminD3Tropfen(wieviel?)

77jährigerPatient- Status

¨ LeichtreduzierterAllgemeinzustand¨ Notfallstation:

¤ Temperatur38.0°C,Blutdruck164/85mmHg,Puls100/min,Sättigung95%ohneO2,GCS15,Atemfrequenz22/min

¤ LeichtezeitlicheundörtlicheDesorientiertheit¤ KörperlicheSchwäche; wirdoffensichtlichbeimAufsitzen/Aufstehen

¤ Gesichtleichtgerötet,leichteSchmerzensakralunduntereBWS

¤ Keinefokal-neurologischenAusfälle¤ InternistischerStatussonstunauffällig

77jährigerPatient- Diagnostik

¨ EKG:Grenzwertigtachykarder Sinusrhythmus¨ RöntgenThorax:KeineAuffälligkeiten¨ Labor:

¤ VenöseBlutgasanalyse:Unauffällig¤ Kreatinin,Elektrolyte,Leberwerte:Normwertig¤ CRP21.5mg/l(Normbis10)¤ LDH393U/l(Normbis225),CK436U/l(50-200)¤ Leukozyten:12.78x109/l(10.99Neutrophile),Lymphozyten0.831x109/l,Hämoglobin181g/l,Thrombozyten327x109/l

¤ Urinstatus:Unauffällig

1. Austritt(keineweiterenAbklärungen)

2. BeginnAntibiotikum(empirischbeiInfektzeichenohneklarenFokus,DDpulmonal)

3. Infusion(HydrierungbeiV.a.Exsikkose)

4. WeitereDiagnostik

5. Punkt2und3

20%

20%

20%

20%

20%

StandardWastun?

020 000

WeitereDiagnostik

¨ CTSchädel:KeineintrakranielleBlutung.Alte,demarkierteInfarktarealeinbeidenMediastromgebieten.FleckigeLeukenzephalopathie, amehestenvaskulärbedingt.

¨ RöntgenBeckenübersicht,Ossacrum,Brustwirbelsäule:KeinHinweisaufFraktur

1. Austritt

2. BeginnAntibiotikum(empirischbeiInfektzeichenohneklarenFokus)

3. Infusion(HydrierungbeiV.a.Exsikkose)

4. WeitereDiagnostik

5. Beobachten

20%

20%

20%

20%

20%

StandardWastun?

020 000

InfusionundstationäreAufnahmemitfolgendemAuftrag:

¨ ErneuteLaborkontrolle:BeierneutemFieber/CRP-Anstiegevtl.Antibiotikaevaluieren

¨ AnmeldungPhysiotherapie¨ JenachneurologischerEvaluation,weiteresProcederediskutieren

¨ ggf.DemenzabklärungimVerlauf

Laborkontrolle

¨ Kreatinin,Elektrolyteunauffällig¨ CRP85.7mg/l(steigend)¨ CK830U/l(50-200),CK-MB-Masse5.4µg/l(<5.0),LDH366U/l(135-225)

¨ Leukozyten9.04x109/l¨ Hämoglobin186g/l,Hämatokrit0.57l/l,MCV101fl(Normbis95)

¨ Thrombozyten322x109/l¨ VitaminB12undErythrozyten-Folsäurenormwertig¨ RestimWesentlichenunauffällig,keineHypoxämie

1. Patienterneutanschauen(Volumenstatus?),Trink(Fremd-)anamnese

2. BeginnAntibiotikum(empirischbeiInfektzeichenohneklarenFokus)

3. Infusion(HydrierungbeiV.a.Exsikkose)

4. Aderlass(Hämatokrit57%)

5. Alles

20%

20%

20%

20%

20%

StandardWastun?

020 000

Alles,nurkeinAntibiotikum…

¨ Volumenstatus:Klinischeuvoläm¨ Fremdanamnese(Ehefrau):Patienttrinktsehrwenig(seitca.1Jahrca.0.5l/Tag),daherweitermitInfusion

¨ Isovolämer Aderlass(V.a.neurologischeVerschlechterungbeiHkt >55%ohneZeichenderHypoxämie)

¨ AbklärungPolyglobulie

DifferentialdiagnosederPolyglobulie

¨ Exsikkose/Diuretika¨ ChronischeHypoxämie(Lungenerkrankungen,rechts-linksShunt,Schlafapnoe,Adipositas-Hypoventilationssyndrom,grosseHöhe,…)

¨ Androgene,anaboleSteroide¨ Doping(Epo)¨ Erythropoietin-produzierendeNeoplasien (Nierenzell-,Hepatozelluläres Karzinom,…)

¨ Erythropoietin-produzierendeNierenläsionennachTransplantation

¨ Knochenmarkserkrankungen(PrimärePolyglobulie,z.B.Polycythämia vera)

Abklärungen

¨ KeineverdächtigenMedikamente/Dopingetc.¨ Polyglobulie persistierttrotzInfusion,Trinkaufforderung,klinischEuvolämie

¨ SauerstoffsättigungauchunterBelastunggut,UnauffälligearterielleBlutgasanalyse,Lungenfunktionunauffällig

¨ Ultraschall-Abdomen:KeineHepatosplenomegalie,Nierenunauffällig,keineRaumforderungen(guteUntersuchungsbedingungen)

¨ Erythropoietin:2.6U/l(Norm4.3- 29.0)

Verdachtsdiagnose

Myeloproliferative Neoplasie vomTypPolycythämia vera

Polycythämia vera

¨ HäufigsteprimärePolycythämie¨ TeilderPhiladelphia-Chromosomnegativenmyeloproliferativen Neoplasien

¨ Amhäufigsten(>90%)durchJAK2 V617FMutation(erworben)– führtzurErythropoietin-Unabhängigkeitbzw.Hypersensitivität

¨ SeltenandereMutationen(JAK2 exon 12Mutationen,andere)

UnserPatient

¨ FazialePlethora,Hypertension,V.a.gestörteMikrozirkulation(neurologischeSymptome)

Major(WHO2016)¨ Hämoglobin>165g/l,Hämatokrit>49%¨ TypischerKnochenmarksbefund¨ PositivfürJAK2 V617F(Exon 14)und JAK2 Exon 12Mutation

Minor¨ Serum-Erythropoietin 2.6IU/l(Norm4.3-29.0)

TherapieoptionenbeiPolycythämia vera

¨ Ziele:Symptomenkontrolle,Lebensverlängerung

¨ Ziel-Hämatokrit<45%,beiFrauenehernochtiefer

¨ DazuAderlässe(z.B.250-500ml)und/oder¨ Zytoreduktive TherapiebeiHochrisikopatienten(z.B.Hydroxyurea)

¨ NiedrigdosiertesAspirin(Thromboseprävention,aberauchhilfreichbeiJuckreizundErythromelalgie)

0 5 1 0 1 5

1 4 0

1 5 0

1 6 0

1 7 0

1 8 0

1 9 0

2 0 0

0 .4 0

0 .4 5

0 .5 0

0 .5 5

0 .6 0

T a g e

Hä

mo

glo

bin

in g

/l Hä

ma

tok

rit in l/l

H äm og lob in

H äm a tok rit

A d e r lä s s e

Verlauf

‘Takehome messages’

¨ AuchbeiälterenMenschenkanndieSturzabklärungselteneUrsachenhervorbringen.

(Problemderawareness)¨ RelevanzderklinischenBefundenichtunterschätzen(beiunseremPatienten:VolumenstatusundfazialePlethora,aberauchdie(Fremd-)Anamnese!)

¨ Erhobene(Labor-)Befundeansehen(VorteilversusRisikobeiunselektiven «Screenings»).

¨ NichtnurdieAnämie,sondernauch(seltener)diePolyglobulie kanneinProblemsein.

Polycythämia vera – häufigeBefunde

¨ Polyglobulie (Hämoglobin,Hämatokrit)¨ Evtl.Leukozytose,Thrombozytose¨ FazialePlethora¨ Juckreiz(evtl.verschlimmertdurch(warmes)Wasser)¨ Geschichtevonthromboembolischen Ereignissen¨ Gicht¨ Bluthochdruck¨ Splenomegalie¨ Sehstörungen,Kopfschmerzen,Schwindel,…

BegrenzterAderlass

¨ BeiSymptomeninfolgevonerhöhtemBlutvolumenund/oderHyperviskosität.

¨ CavebeiPatientenmitchronischerHypoxämie¨ Initial250ml(isovoläm),sonstbis500ml¨ 500mlsenkendenHämatokritumca.3%¨ TypischeIndikationen:Polycythämia vera,Hämochromatose,Porphyria cutanea tarda

JAK2MPLCALR

EssentialThrombocythemia (ET)

Platelets

Nature Reviews | Cancer

Neutrophils

Monocytes

Myeloid progenitor

HSC

MPD Activating mutation

Chronic myeloidleukaemiaChronic myelomonocyticleukaemiaPrimarymyelofibrosis

Mast cell

Systemicmastocytosis

KITD816VFIP1L1–PDGFRA

Red blood cells

Polycythaemiavera

JAK2V617FJAK2 Exon 12

Platelets Essentialthrombo-cythaemia

JAK2V617FMPLW515L/K

JAK2V617FMPLW515L/K

Eosinophils Chroniceosinophilicleukemia

FIP1L1–PDGFRA

BCR-ABL

TEL–PDGFRBBCR–PDGFRATEL–JAK2other fusion TKs

LeucopaeniaLow white blood cell count.

ErythrocytosisIncreased red blood cell count.

In vitro studies demonstrate that the expression of JAK2V617F activates multiple downstream signalling pathways8,36, including the Stat family of transcription factors, the mitogen activated protein kinase (MAPK) signalling pathway, and the phosphotidylinositol 3-kinase (PI3K)–Akt signalling pathway (FIG. 2). Most activated tyrosine kinases that have been identified in human malignancies activate these same signalling cascades, and the role and requirement of the Stat, MAPK and PI3K–Akt signalling pathways in JAK2V617F-mediated transformation of haematopoietic cells has not been fully elucidated. However, several lines of evidence suggest that activation of the Stat family of transcription factors is important in JAK2V617F-mediated transformation.

First, expression of either constitutively active STAT5 or its anti-apoptotic target gene BCL-XL in human haemat-opoietic progenitors results in EPO-independent colony formation37, a hallmark of human PV. Moreover, STAT3 activation and BCL-XL overexpression are observed in most PV patient samples38,39. These data imply that Stat pathway activation is important in JAK2V617F-mediated transformation, but do not indicate whether Stat pathway activation is necessary and/or sufficient for JAK2V617F-mediated transformation. Murine bone marrow trans-plantation (BMT) assays using Stat5a;Stat5b-deficient mice have been used to show that STAT5 is required for haematopoietic transformation by the constitutively active TEL–JAK2 fusion tyrosine kinase40, and future experiments will ultimately determine whether the same is true for JAK2V617F. In addition, the activation of sig-nalling by the JAK2V617F kinase might in part be due to escape from negative-feedback mechanisms important in attenuating JAK2 signalling. Jak activity is negatively regulated by the Socs family of proteins, which normally bind to the Jak kinases and result in their degradation. In particular, SOCS1 and SOCS3 have been shown to bind to JAK2 and inhibit JAK2 catalytic activity41,42, and over-expression of SOCS1 results in abrogation of in vitro and in vivo transformation by TEL–JAK2 (REF. 43). Although expression of SOCS1 results in JAK2 and JAK2V617F deg-radation and inhibition of kinase activity, the expression of SOCS3 paradoxically results in increased JAK2V617F protein stability, increased SOCS3 phosphorylation and increased JAK2V617F phosphorylation44. These data demonstrate that regulation of JAK2 kinase activity by SOCS3 is altered in the context of the V617F substitution, and suggest the possibility that therapeutic inhibition of SOCS3 might selectively attenuate JAK2V617F, but not wild-type JAK2 signalling.

In vivo data from murine BMT experiments have provided important insights into the role of JAK2 activa-tion in the pathogenesis of MPD. James et al. noted that the expression of JAK2V617F, but not wild-type JAK2, in a murine BMT assay results in significant erythrocytosis in recipient mice 28 days after transplantation8,9, and

Figure 1 | Classification and molecular pathogenesis of the MPD. The different myeloproliferative disorders (MPD) can be classified by the predominant terminally differentiated myeloid cell involved in the disorder, and for each terminally differentiated myeloid cell there is a clinically distinct MPD. Different approaches have been used to identify the activating alleles that cause these disorders, and in all cases these alleles result in constitutive tyrosine kinase signalling. HSC, haematopoietic stem cell; JAK2, Janus kinase 2; MPL, thrombopoietin receptor; PDGFR, platelet derived growth factor receptor.

Box 1 | Myeloproliferative diseases

Myeloproliferative diseases (MPD) are proliferative syndromes that present with increased numbers of functional, mature, terminally differentiated myeloid elements. Thus, in contrast to impaired myeloid differentiation that is characteristic of myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML), patients with MPD rarely present with infectious complications related to leucopaenia, or complications related to anaemia that are observed in MDS and AML. In addition, although these MPD can be associated with bleeding tendencies (diatheses) that are observed in MDS and AML owing to thrombocytosis and platelet dysfunction, they are often associated with specific clinical sequelae that include thrombotic diatheses that are not observed in MDS or AML. However, MDS and AML are thought to be stem cell disorders, based on clonality analyses using X-inactivation-based assays in females or cytogenetic analyses that demonstrate the clonal involvement of the different haematopoietic lineages111,112.

The first MPD allele to be identified and characterized was the BCR-ABL fusion oncogene in chronic myeloid leukaemia (CML) that arises as a consequence of the 9:22 translocation2, a causative agent for this disease, and recurrent translocations led to the discovery of different fusion tyrosine kinases, including platelet derived growth factor receptor-B (PDGFRB) fusions in patients with chronic myelomonocytic leukaemia (CMML) and other MPD3. The observation that KIT is highly expressed in mast cells led to the discovery of the KITD816V allele in systemic mastocytosis (SM)5, and imatinib responses in patients with chronic eosinophilic leukaemia (CEL) and SM led to the discovery of an interstitial deletion that gives rise to the FIP1L1–PDGFRA fusion4. Most recently, investigators have identified Janus kinase 2 (JAK2) and thrombopoietin receptor mutations in polycythaemia vera (PV), essential thrombocythaemia (ET) and primary meylofibrosis (PMF) that result in the activation of JAK2 signalling8–11,53,55.

R E V I E W S

NATURE REVIEWS | CANCER VOLUME 7 | SEPTEMBER 2007 | 675

Nature Reviews | Cancer

Neutrophils

Monocytes

Myeloid progenitor

HSC

MPD Activating mutation

Chronic myeloidleukaemiaChronic myelomonocyticleukaemiaPrimarymyelofibrosis

Mast cell

Systemicmastocytosis

KITD816VFIP1L1–PDGFRA

Red blood cells

Polycythaemiavera

JAK2V617FJAK2 Exon 12

Platelets Essentialthrombo-cythaemia

JAK2V617FMPLW515L/K

JAK2V617FMPLW515L/K

Eosinophils Chroniceosinophilicleukemia

FIP1L1–PDGFRA

BCR-ABL

TEL–PDGFRBBCR–PDGFRATEL–JAK2other fusion TKs

LeucopaeniaLow white blood cell count.

ErythrocytosisIncreased red blood cell count.

In vitro studies demonstrate that the expression of JAK2V617F activates multiple downstream signalling pathways8,36, including the Stat family of transcription factors, the mitogen activated protein kinase (MAPK) signalling pathway, and the phosphotidylinositol 3-kinase (PI3K)–Akt signalling pathway (FIG. 2). Most activated tyrosine kinases that have been identified in human malignancies activate these same signalling cascades, and the role and requirement of the Stat, MAPK and PI3K–Akt signalling pathways in JAK2V617F-mediated transformation of haematopoietic cells has not been fully elucidated. However, several lines of evidence suggest that activation of the Stat family of transcription factors is important in JAK2V617F-mediated transformation.

First, expression of either constitutively active STAT5 or its anti-apoptotic target gene BCL-XL in human haemat-opoietic progenitors results in EPO-independent colony formation37, a hallmark of human PV. Moreover, STAT3 activation and BCL-XL overexpression are observed in most PV patient samples38,39. These data imply that Stat pathway activation is important in JAK2V617F-mediated transformation, but do not indicate whether Stat pathway activation is necessary and/or sufficient for JAK2V617F-mediated transformation. Murine bone marrow trans-plantation (BMT) assays using Stat5a;Stat5b-deficient mice have been used to show that STAT5 is required for haematopoietic transformation by the constitutively active TEL–JAK2 fusion tyrosine kinase40, and future experiments will ultimately determine whether the same is true for JAK2V617F. In addition, the activation of sig-nalling by the JAK2V617F kinase might in part be due to escape from negative-feedback mechanisms important in attenuating JAK2 signalling. Jak activity is negatively regulated by the Socs family of proteins, which normally bind to the Jak kinases and result in their degradation. In particular, SOCS1 and SOCS3 have been shown to bind to JAK2 and inhibit JAK2 catalytic activity41,42, and over-expression of SOCS1 results in abrogation of in vitro and in vivo transformation by TEL–JAK2 (REF. 43). Although expression of SOCS1 results in JAK2 and JAK2V617F deg-radation and inhibition of kinase activity, the expression of SOCS3 paradoxically results in increased JAK2V617F protein stability, increased SOCS3 phosphorylation and increased JAK2V617F phosphorylation44. These data demonstrate that regulation of JAK2 kinase activity by SOCS3 is altered in the context of the V617F substitution, and suggest the possibility that therapeutic inhibition of SOCS3 might selectively attenuate JAK2V617F, but not wild-type JAK2 signalling.

In vivo data from murine BMT experiments have provided important insights into the role of JAK2 activa-tion in the pathogenesis of MPD. James et al. noted that the expression of JAK2V617F, but not wild-type JAK2, in a murine BMT assay results in significant erythrocytosis in recipient mice 28 days after transplantation8,9, and

Figure 1 | Classification and molecular pathogenesis of the MPD. The different myeloproliferative disorders (MPD) can be classified by the predominant terminally differentiated myeloid cell involved in the disorder, and for each terminally differentiated myeloid cell there is a clinically distinct MPD. Different approaches have been used to identify the activating alleles that cause these disorders, and in all cases these alleles result in constitutive tyrosine kinase signalling. HSC, haematopoietic stem cell; JAK2, Janus kinase 2; MPL, thrombopoietin receptor; PDGFR, platelet derived growth factor receptor.

Box 1 | Myeloproliferative diseases

Myeloproliferative diseases (MPD) are proliferative syndromes that present with increased numbers of functional, mature, terminally differentiated myeloid elements. Thus, in contrast to impaired myeloid differentiation that is characteristic of myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML), patients with MPD rarely present with infectious complications related to leucopaenia, or complications related to anaemia that are observed in MDS and AML. In addition, although these MPD can be associated with bleeding tendencies (diatheses) that are observed in MDS and AML owing to thrombocytosis and platelet dysfunction, they are often associated with specific clinical sequelae that include thrombotic diatheses that are not observed in MDS or AML. However, MDS and AML are thought to be stem cell disorders, based on clonality analyses using X-inactivation-based assays in females or cytogenetic analyses that demonstrate the clonal involvement of the different haematopoietic lineages111,112.

The first MPD allele to be identified and characterized was the BCR-ABL fusion oncogene in chronic myeloid leukaemia (CML) that arises as a consequence of the 9:22 translocation2, a causative agent for this disease, and recurrent translocations led to the discovery of different fusion tyrosine kinases, including platelet derived growth factor receptor-B (PDGFRB) fusions in patients with chronic myelomonocytic leukaemia (CMML) and other MPD3. The observation that KIT is highly expressed in mast cells led to the discovery of the KITD816V allele in systemic mastocytosis (SM)5, and imatinib responses in patients with chronic eosinophilic leukaemia (CEL) and SM led to the discovery of an interstitial deletion that gives rise to the FIP1L1–PDGFRA fusion4. Most recently, investigators have identified Janus kinase 2 (JAK2) and thrombopoietin receptor mutations in polycythaemia vera (PV), essential thrombocythaemia (ET) and primary meylofibrosis (PMF) that result in the activation of JAK2 signalling8–11,53,55.

R E V I E W S

NATURE REVIEWS | CANCER VOLUME 7 | SEPTEMBER 2007 | 675

Myelofibrosis (MF)Bonemarrowfibrosis

Nature Reviews | Cancer

Neutrophils

Monocytes

Myeloid progenitor

HSC

MPD Activating mutation

Chronic myeloidleukaemiaChronic myelomonocyticleukaemiaPrimarymyelofibrosis

Mast cell

Systemicmastocytosis

KITD816VFIP1L1–PDGFRA

Red blood cells

Polycythaemiavera

JAK2V617FJAK2 Exon 12

Platelets Essentialthrombo-cythaemia

JAK2V617FMPLW515L/K

JAK2V617FMPLW515L/K

Eosinophils Chroniceosinophilicleukemia

FIP1L1–PDGFRA

BCR-ABL

TEL–PDGFRBBCR–PDGFRATEL–JAK2other fusion TKs

LeucopaeniaLow white blood cell count.

ErythrocytosisIncreased red blood cell count.

In vitro studies demonstrate that the expression of JAK2V617F activates multiple downstream signalling pathways8,36, including the Stat family of transcription factors, the mitogen activated protein kinase (MAPK) signalling pathway, and the phosphotidylinositol 3-kinase (PI3K)–Akt signalling pathway (FIG. 2). Most activated tyrosine kinases that have been identified in human malignancies activate these same signalling cascades, and the role and requirement of the Stat, MAPK and PI3K–Akt signalling pathways in JAK2V617F-mediated transformation of haematopoietic cells has not been fully elucidated. However, several lines of evidence suggest that activation of the Stat family of transcription factors is important in JAK2V617F-mediated transformation.

First, expression of either constitutively active STAT5 or its anti-apoptotic target gene BCL-XL in human haemat-opoietic progenitors results in EPO-independent colony formation37, a hallmark of human PV. Moreover, STAT3 activation and BCL-XL overexpression are observed in most PV patient samples38,39. These data imply that Stat pathway activation is important in JAK2V617F-mediated transformation, but do not indicate whether Stat pathway activation is necessary and/or sufficient for JAK2V617F-mediated transformation. Murine bone marrow trans-plantation (BMT) assays using Stat5a;Stat5b-deficient mice have been used to show that STAT5 is required for haematopoietic transformation by the constitutively active TEL–JAK2 fusion tyrosine kinase40, and future experiments will ultimately determine whether the same is true for JAK2V617F. In addition, the activation of sig-nalling by the JAK2V617F kinase might in part be due to escape from negative-feedback mechanisms important in attenuating JAK2 signalling. Jak activity is negatively regulated by the Socs family of proteins, which normally bind to the Jak kinases and result in their degradation. In particular, SOCS1 and SOCS3 have been shown to bind to JAK2 and inhibit JAK2 catalytic activity41,42, and over-expression of SOCS1 results in abrogation of in vitro and in vivo transformation by TEL–JAK2 (REF. 43). Although expression of SOCS1 results in JAK2 and JAK2V617F deg-radation and inhibition of kinase activity, the expression of SOCS3 paradoxically results in increased JAK2V617F protein stability, increased SOCS3 phosphorylation and increased JAK2V617F phosphorylation44. These data demonstrate that regulation of JAK2 kinase activity by SOCS3 is altered in the context of the V617F substitution, and suggest the possibility that therapeutic inhibition of SOCS3 might selectively attenuate JAK2V617F, but not wild-type JAK2 signalling.

In vivo data from murine BMT experiments have provided important insights into the role of JAK2 activa-tion in the pathogenesis of MPD. James et al. noted that the expression of JAK2V617F, but not wild-type JAK2, in a murine BMT assay results in significant erythrocytosis in recipient mice 28 days after transplantation8,9, and

Figure 1 | Classification and molecular pathogenesis of the MPD. The different myeloproliferative disorders (MPD) can be classified by the predominant terminally differentiated myeloid cell involved in the disorder, and for each terminally differentiated myeloid cell there is a clinically distinct MPD. Different approaches have been used to identify the activating alleles that cause these disorders, and in all cases these alleles result in constitutive tyrosine kinase signalling. HSC, haematopoietic stem cell; JAK2, Janus kinase 2; MPL, thrombopoietin receptor; PDGFR, platelet derived growth factor receptor.

Box 1 | Myeloproliferative diseases

Myeloproliferative diseases (MPD) are proliferative syndromes that present with increased numbers of functional, mature, terminally differentiated myeloid elements. Thus, in contrast to impaired myeloid differentiation that is characteristic of myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML), patients with MPD rarely present with infectious complications related to leucopaenia, or complications related to anaemia that are observed in MDS and AML. In addition, although these MPD can be associated with bleeding tendencies (diatheses) that are observed in MDS and AML owing to thrombocytosis and platelet dysfunction, they are often associated with specific clinical sequelae that include thrombotic diatheses that are not observed in MDS or AML. However, MDS and AML are thought to be stem cell disorders, based on clonality analyses using X-inactivation-based assays in females or cytogenetic analyses that demonstrate the clonal involvement of the different haematopoietic lineages111,112.

The first MPD allele to be identified and characterized was the BCR-ABL fusion oncogene in chronic myeloid leukaemia (CML) that arises as a consequence of the 9:22 translocation2, a causative agent for this disease, and recurrent translocations led to the discovery of different fusion tyrosine kinases, including platelet derived growth factor receptor-B (PDGFRB) fusions in patients with chronic myelomonocytic leukaemia (CMML) and other MPD3. The observation that KIT is highly expressed in mast cells led to the discovery of the KITD816V allele in systemic mastocytosis (SM)5, and imatinib responses in patients with chronic eosinophilic leukaemia (CEL) and SM led to the discovery of an interstitial deletion that gives rise to the FIP1L1–PDGFRA fusion4. Most recently, investigators have identified Janus kinase 2 (JAK2) and thrombopoietin receptor mutations in polycythaemia vera (PV), essential thrombocythaemia (ET) and primary meylofibrosis (PMF) that result in the activation of JAK2 signalling8–11,53,55.

R E V I E W S

NATURE REVIEWS | CANCER VOLUME 7 | SEPTEMBER 2007 | 675

Redbloodcells

Nature Reviews | Cancer

Neutrophils

Monocytes

Myeloid progenitor

HSC

MPD Activating mutation

Chronic myeloidleukaemiaChronic myelomonocyticleukaemiaPrimarymyelofibrosis

Mast cell

Systemicmastocytosis

KITD816VFIP1L1–PDGFRA

Red blood cells

Polycythaemiavera

JAK2V617FJAK2 Exon 12

Platelets Essentialthrombo-cythaemia

JAK2V617FMPLW515L/K

JAK2V617FMPLW515L/K

Eosinophils Chroniceosinophilicleukemia

FIP1L1–PDGFRA

BCR-ABL

TEL–PDGFRBBCR–PDGFRATEL–JAK2other fusion TKs

LeucopaeniaLow white blood cell count.

ErythrocytosisIncreased red blood cell count.

In vitro studies demonstrate that the expression of JAK2V617F activates multiple downstream signalling pathways8,36, including the Stat family of transcription factors, the mitogen activated protein kinase (MAPK) signalling pathway, and the phosphotidylinositol 3-kinase (PI3K)–Akt signalling pathway (FIG. 2). Most activated tyrosine kinases that have been identified in human malignancies activate these same signalling cascades, and the role and requirement of the Stat, MAPK and PI3K–Akt signalling pathways in JAK2V617F-mediated transformation of haematopoietic cells has not been fully elucidated. However, several lines of evidence suggest that activation of the Stat family of transcription factors is important in JAK2V617F-mediated transformation.

First, expression of either constitutively active STAT5 or its anti-apoptotic target gene BCL-XL in human haemat-opoietic progenitors results in EPO-independent colony formation37, a hallmark of human PV. Moreover, STAT3 activation and BCL-XL overexpression are observed in most PV patient samples38,39. These data imply that Stat pathway activation is important in JAK2V617F-mediated transformation, but do not indicate whether Stat pathway activation is necessary and/or sufficient for JAK2V617F-mediated transformation. Murine bone marrow trans-plantation (BMT) assays using Stat5a;Stat5b-deficient mice have been used to show that STAT5 is required for haematopoietic transformation by the constitutively active TEL–JAK2 fusion tyrosine kinase40, and future experiments will ultimately determine whether the same is true for JAK2V617F. In addition, the activation of sig-nalling by the JAK2V617F kinase might in part be due to escape from negative-feedback mechanisms important in attenuating JAK2 signalling. Jak activity is negatively regulated by the Socs family of proteins, which normally bind to the Jak kinases and result in their degradation. In particular, SOCS1 and SOCS3 have been shown to bind to JAK2 and inhibit JAK2 catalytic activity41,42, and over-expression of SOCS1 results in abrogation of in vitro and in vivo transformation by TEL–JAK2 (REF. 43). Although expression of SOCS1 results in JAK2 and JAK2V617F deg-radation and inhibition of kinase activity, the expression of SOCS3 paradoxically results in increased JAK2V617F protein stability, increased SOCS3 phosphorylation and increased JAK2V617F phosphorylation44. These data demonstrate that regulation of JAK2 kinase activity by SOCS3 is altered in the context of the V617F substitution, and suggest the possibility that therapeutic inhibition of SOCS3 might selectively attenuate JAK2V617F, but not wild-type JAK2 signalling.

In vivo data from murine BMT experiments have provided important insights into the role of JAK2 activa-tion in the pathogenesis of MPD. James et al. noted that the expression of JAK2V617F, but not wild-type JAK2, in a murine BMT assay results in significant erythrocytosis in recipient mice 28 days after transplantation8,9, and

Figure 1 | Classification and molecular pathogenesis of the MPD. The different myeloproliferative disorders (MPD) can be classified by the predominant terminally differentiated myeloid cell involved in the disorder, and for each terminally differentiated myeloid cell there is a clinically distinct MPD. Different approaches have been used to identify the activating alleles that cause these disorders, and in all cases these alleles result in constitutive tyrosine kinase signalling. HSC, haematopoietic stem cell; JAK2, Janus kinase 2; MPL, thrombopoietin receptor; PDGFR, platelet derived growth factor receptor.

Box 1 | Myeloproliferative diseases

Myeloproliferative diseases (MPD) are proliferative syndromes that present with increased numbers of functional, mature, terminally differentiated myeloid elements. Thus, in contrast to impaired myeloid differentiation that is characteristic of myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML), patients with MPD rarely present with infectious complications related to leucopaenia, or complications related to anaemia that are observed in MDS and AML. In addition, although these MPD can be associated with bleeding tendencies (diatheses) that are observed in MDS and AML owing to thrombocytosis and platelet dysfunction, they are often associated with specific clinical sequelae that include thrombotic diatheses that are not observed in MDS or AML. However, MDS and AML are thought to be stem cell disorders, based on clonality analyses using X-inactivation-based assays in females or cytogenetic analyses that demonstrate the clonal involvement of the different haematopoietic lineages111,112.

The first MPD allele to be identified and characterized was the BCR-ABL fusion oncogene in chronic myeloid leukaemia (CML) that arises as a consequence of the 9:22 translocation2, a causative agent for this disease, and recurrent translocations led to the discovery of different fusion tyrosine kinases, including platelet derived growth factor receptor-B (PDGFRB) fusions in patients with chronic myelomonocytic leukaemia (CMML) and other MPD3. The observation that KIT is highly expressed in mast cells led to the discovery of the KITD816V allele in systemic mastocytosis (SM)5, and imatinib responses in patients with chronic eosinophilic leukaemia (CEL) and SM led to the discovery of an interstitial deletion that gives rise to the FIP1L1–PDGFRA fusion4. Most recently, investigators have identified Janus kinase 2 (JAK2) and thrombopoietin receptor mutations in polycythaemia vera (PV), essential thrombocythaemia (ET) and primary meylofibrosis (PMF) that result in the activation of JAK2 signalling8–11,53,55.

R E V I E W S

NATURE REVIEWS | CANCER VOLUME 7 | SEPTEMBER 2007 | 675

Nature Reviews | Cancer

Neutrophils

Monocytes

Myeloid progenitor

HSC

MPD Activating mutation

Chronic myeloidleukaemiaChronic myelomonocyticleukaemiaPrimarymyelofibrosis

Mast cell

Systemicmastocytosis

KITD816VFIP1L1–PDGFRA

Red blood cells

Polycythaemiavera

JAK2V617FJAK2 Exon 12

Platelets Essentialthrombo-cythaemia

JAK2V617FMPLW515L/K

JAK2V617FMPLW515L/K

Eosinophils Chroniceosinophilicleukemia

FIP1L1–PDGFRA

BCR-ABL

TEL–PDGFRBBCR–PDGFRATEL–JAK2other fusion TKs

LeucopaeniaLow white blood cell count.

ErythrocytosisIncreased red blood cell count.

In vitro studies demonstrate that the expression of JAK2V617F activates multiple downstream signalling pathways8,36, including the Stat family of transcription factors, the mitogen activated protein kinase (MAPK) signalling pathway, and the phosphotidylinositol 3-kinase (PI3K)–Akt signalling pathway (FIG. 2). Most activated tyrosine kinases that have been identified in human malignancies activate these same signalling cascades, and the role and requirement of the Stat, MAPK and PI3K–Akt signalling pathways in JAK2V617F-mediated transformation of haematopoietic cells has not been fully elucidated. However, several lines of evidence suggest that activation of the Stat family of transcription factors is important in JAK2V617F-mediated transformation.

First, expression of either constitutively active STAT5 or its anti-apoptotic target gene BCL-XL in human haemat-opoietic progenitors results in EPO-independent colony formation37, a hallmark of human PV. Moreover, STAT3 activation and BCL-XL overexpression are observed in most PV patient samples38,39. These data imply that Stat pathway activation is important in JAK2V617F-mediated transformation, but do not indicate whether Stat pathway activation is necessary and/or sufficient for JAK2V617F-mediated transformation. Murine bone marrow trans-plantation (BMT) assays using Stat5a;Stat5b-deficient mice have been used to show that STAT5 is required for haematopoietic transformation by the constitutively active TEL–JAK2 fusion tyrosine kinase40, and future experiments will ultimately determine whether the same is true for JAK2V617F. In addition, the activation of sig-nalling by the JAK2V617F kinase might in part be due to escape from negative-feedback mechanisms important in attenuating JAK2 signalling. Jak activity is negatively regulated by the Socs family of proteins, which normally bind to the Jak kinases and result in their degradation. In particular, SOCS1 and SOCS3 have been shown to bind to JAK2 and inhibit JAK2 catalytic activity41,42, and over-expression of SOCS1 results in abrogation of in vitro and in vivo transformation by TEL–JAK2 (REF. 43). Although expression of SOCS1 results in JAK2 and JAK2V617F deg-radation and inhibition of kinase activity, the expression of SOCS3 paradoxically results in increased JAK2V617F protein stability, increased SOCS3 phosphorylation and increased JAK2V617F phosphorylation44. These data demonstrate that regulation of JAK2 kinase activity by SOCS3 is altered in the context of the V617F substitution, and suggest the possibility that therapeutic inhibition of SOCS3 might selectively attenuate JAK2V617F, but not wild-type JAK2 signalling.

In vivo data from murine BMT experiments have provided important insights into the role of JAK2 activa-tion in the pathogenesis of MPD. James et al. noted that the expression of JAK2V617F, but not wild-type JAK2, in a murine BMT assay results in significant erythrocytosis in recipient mice 28 days after transplantation8,9, and

Figure 1 | Classification and molecular pathogenesis of the MPD. The different myeloproliferative disorders (MPD) can be classified by the predominant terminally differentiated myeloid cell involved in the disorder, and for each terminally differentiated myeloid cell there is a clinically distinct MPD. Different approaches have been used to identify the activating alleles that cause these disorders, and in all cases these alleles result in constitutive tyrosine kinase signalling. HSC, haematopoietic stem cell; JAK2, Janus kinase 2; MPL, thrombopoietin receptor; PDGFR, platelet derived growth factor receptor.

Box 1 | Myeloproliferative diseases

Myeloproliferative diseases (MPD) are proliferative syndromes that present with increased numbers of functional, mature, terminally differentiated myeloid elements. Thus, in contrast to impaired myeloid differentiation that is characteristic of myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML), patients with MPD rarely present with infectious complications related to leucopaenia, or complications related to anaemia that are observed in MDS and AML. In addition, although these MPD can be associated with bleeding tendencies (diatheses) that are observed in MDS and AML owing to thrombocytosis and platelet dysfunction, they are often associated with specific clinical sequelae that include thrombotic diatheses that are not observed in MDS or AML. However, MDS and AML are thought to be stem cell disorders, based on clonality analyses using X-inactivation-based assays in females or cytogenetic analyses that demonstrate the clonal involvement of the different haematopoietic lineages111,112.

The first MPD allele to be identified and characterized was the BCR-ABL fusion oncogene in chronic myeloid leukaemia (CML) that arises as a consequence of the 9:22 translocation2, a causative agent for this disease, and recurrent translocations led to the discovery of different fusion tyrosine kinases, including platelet derived growth factor receptor-B (PDGFRB) fusions in patients with chronic myelomonocytic leukaemia (CMML) and other MPD3. The observation that KIT is highly expressed in mast cells led to the discovery of the KITD816V allele in systemic mastocytosis (SM)5, and imatinib responses in patients with chronic eosinophilic leukaemia (CEL) and SM led to the discovery of an interstitial deletion that gives rise to the FIP1L1–PDGFRA fusion4. Most recently, investigators have identified Janus kinase 2 (JAK2) and thrombopoietin receptor mutations in polycythaemia vera (PV), essential thrombocythaemia (ET) and primary meylofibrosis (PMF) that result in the activation of JAK2 signalling8–11,53,55.

R E V I E W S

NATURE REVIEWS | CANCER VOLUME 7 | SEPTEMBER 2007 | 675

Nature Reviews | Cancer

Neutrophils

Monocytes

Myeloid progenitor

HSC

MPD Activating mutation

Chronic myeloidleukaemiaChronic myelomonocyticleukaemiaPrimarymyelofibrosis

Mast cell

Systemicmastocytosis

KITD816VFIP1L1–PDGFRA

Red blood cells

Polycythaemiavera

JAK2V617FJAK2 Exon 12

Platelets Essentialthrombo-cythaemia

JAK2V617FMPLW515L/K

JAK2V617FMPLW515L/K

Eosinophils Chroniceosinophilicleukemia

FIP1L1–PDGFRA

BCR-ABL

TEL–PDGFRBBCR–PDGFRATEL–JAK2other fusion TKs

LeucopaeniaLow white blood cell count.

ErythrocytosisIncreased red blood cell count.

In vitro studies demonstrate that the expression of JAK2V617F activates multiple downstream signalling pathways8,36, including the Stat family of transcription factors, the mitogen activated protein kinase (MAPK) signalling pathway, and the phosphotidylinositol 3-kinase (PI3K)–Akt signalling pathway (FIG. 2). Most activated tyrosine kinases that have been identified in human malignancies activate these same signalling cascades, and the role and requirement of the Stat, MAPK and PI3K–Akt signalling pathways in JAK2V617F-mediated transformation of haematopoietic cells has not been fully elucidated. However, several lines of evidence suggest that activation of the Stat family of transcription factors is important in JAK2V617F-mediated transformation.

First, expression of either constitutively active STAT5 or its anti-apoptotic target gene BCL-XL in human haemat-opoietic progenitors results in EPO-independent colony formation37, a hallmark of human PV. Moreover, STAT3 activation and BCL-XL overexpression are observed in most PV patient samples38,39. These data imply that Stat pathway activation is important in JAK2V617F-mediated transformation, but do not indicate whether Stat pathway activation is necessary and/or sufficient for JAK2V617F-mediated transformation. Murine bone marrow trans-plantation (BMT) assays using Stat5a;Stat5b-deficient mice have been used to show that STAT5 is required for haematopoietic transformation by the constitutively active TEL–JAK2 fusion tyrosine kinase40, and future experiments will ultimately determine whether the same is true for JAK2V617F. In addition, the activation of sig-nalling by the JAK2V617F kinase might in part be due to escape from negative-feedback mechanisms important in attenuating JAK2 signalling. Jak activity is negatively regulated by the Socs family of proteins, which normally bind to the Jak kinases and result in their degradation. In particular, SOCS1 and SOCS3 have been shown to bind to JAK2 and inhibit JAK2 catalytic activity41,42, and over-expression of SOCS1 results in abrogation of in vitro and in vivo transformation by TEL–JAK2 (REF. 43). Although expression of SOCS1 results in JAK2 and JAK2V617F deg-radation and inhibition of kinase activity, the expression of SOCS3 paradoxically results in increased JAK2V617F protein stability, increased SOCS3 phosphorylation and increased JAK2V617F phosphorylation44. These data demonstrate that regulation of JAK2 kinase activity by SOCS3 is altered in the context of the V617F substitution, and suggest the possibility that therapeutic inhibition of SOCS3 might selectively attenuate JAK2V617F, but not wild-type JAK2 signalling.

In vivo data from murine BMT experiments have provided important insights into the role of JAK2 activa-tion in the pathogenesis of MPD. James et al. noted that the expression of JAK2V617F, but not wild-type JAK2, in a murine BMT assay results in significant erythrocytosis in recipient mice 28 days after transplantation8,9, and

Figure 1 | Classification and molecular pathogenesis of the MPD. The different myeloproliferative disorders (MPD) can be classified by the predominant terminally differentiated myeloid cell involved in the disorder, and for each terminally differentiated myeloid cell there is a clinically distinct MPD. Different approaches have been used to identify the activating alleles that cause these disorders, and in all cases these alleles result in constitutive tyrosine kinase signalling. HSC, haematopoietic stem cell; JAK2, Janus kinase 2; MPL, thrombopoietin receptor; PDGFR, platelet derived growth factor receptor.

Box 1 | Myeloproliferative diseases

Myeloproliferative diseases (MPD) are proliferative syndromes that present with increased numbers of functional, mature, terminally differentiated myeloid elements. Thus, in contrast to impaired myeloid differentiation that is characteristic of myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML), patients with MPD rarely present with infectious complications related to leucopaenia, or complications related to anaemia that are observed in MDS and AML. In addition, although these MPD can be associated with bleeding tendencies (diatheses) that are observed in MDS and AML owing to thrombocytosis and platelet dysfunction, they are often associated with specific clinical sequelae that include thrombotic diatheses that are not observed in MDS or AML. However, MDS and AML are thought to be stem cell disorders, based on clonality analyses using X-inactivation-based assays in females or cytogenetic analyses that demonstrate the clonal involvement of the different haematopoietic lineages111,112.

The first MPD allele to be identified and characterized was the BCR-ABL fusion oncogene in chronic myeloid leukaemia (CML) that arises as a consequence of the 9:22 translocation2, a causative agent for this disease, and recurrent translocations led to the discovery of different fusion tyrosine kinases, including platelet derived growth factor receptor-B (PDGFRB) fusions in patients with chronic myelomonocytic leukaemia (CMML) and other MPD3. The observation that KIT is highly expressed in mast cells led to the discovery of the KITD816V allele in systemic mastocytosis (SM)5, and imatinib responses in patients with chronic eosinophilic leukaemia (CEL) and SM led to the discovery of an interstitial deletion that gives rise to the FIP1L1–PDGFRA fusion4. Most recently, investigators have identified Janus kinase 2 (JAK2) and thrombopoietin receptor mutations in polycythaemia vera (PV), essential thrombocythaemia (ET) and primary meylofibrosis (PMF) that result in the activation of JAK2 signalling8–11,53,55.

R E V I E W S

NATURE REVIEWS | CANCER VOLUME 7 | SEPTEMBER 2007 | 675

PolycythemiaVera(PV)

Nature Reviews | Cancer

Neutrophils

Monocytes

Myeloid progenitor

HSC

MPDActivating mutation

Chronic myeloidleukaemiaChronic myelomonocyticleukaemiaPrimarymyelofibrosis

Mast cell

Systemicmastocytosis

KITD816VFIP1L1–PDGFRA

Red blood cells

Polycythaemiavera

JAK2V617FJAK2 Exon 12

PlateletsEssentialthrombo-cythaemia

JAK2V617FMPLW515L/K

JAK2V617FMPLW515L/K

EosinophilsChroniceosinophilicleukemia

FIP1L1–PDGFRA

BCR-ABL

TEL–PDGFRBBCR–PDGFRATEL–JAK2other fusion TKs

LeucopaeniaLow white blood cell count.

ErythrocytosisIncreased red blood cell count.

In vitro studies demonstrate that the expression of JAK2V617F activates multiple downstream signalling pathways8,36, including the Stat family of transcription factors, the mitogen activated protein kinase (MAPK) signalling pathway, and the phosphotidylinositol 3-kinase (PI3K)–Akt signalling pathway (FIG. 2). Most activated tyrosine kinases that have been identified in human malignancies activate these same signalling cascades, and the role and requirement of the Stat, MAPK and PI3K–Akt signalling pathways in JAK2V617F-mediated transformation of haematopoietic cells has not been fully elucidated. However, several lines of evidence suggest that activation of the Stat family of transcription factors is important in JAK2V617F-mediated transformation.

First, expression of either constitutively active STAT5 or its anti-apoptotic target gene BCL-XL in human haemat-opoietic progenitors results in EPO-independent colony formation37, a hallmark of human PV. Moreover, STAT3 activation and BCL-XL overexpression are observed in most PV patient samples38,39. These data imply that Stat pathway activation is important in JAK2V617F-mediated transformation, but do not indicate whether Stat pathway activation is necessary and/or sufficient for JAK2V617F-mediated transformation. Murine bone marrow trans-plantation (BMT) assays using Stat5a;Stat5b-deficient mice have been used to show that STAT5 is required for haematopoietic transformation by the constitutively active TEL–JAK2 fusion tyrosine kinase40, and future experiments will ultimately determine whether the same is true for JAK2V617F. In addition, the activation of sig-nalling by the JAK2V617F kinase might in part be due to escape from negative-feedback mechanisms important in attenuating JAK2 signalling. Jak activity is negatively regulated by the Socs family of proteins, which normally bind to the Jak kinases and result in their degradation. In particular, SOCS1 and SOCS3 have been shown to bind to JAK2 and inhibit JAK2 catalytic activity41,42, and over-expression of SOCS1 results in abrogation of in vitro and in vivo transformation by TEL–JAK2 (REF. 43). Although expression of SOCS1 results in JAK2 and JAK2V617F deg-radation and inhibition of kinase activity, the expression of SOCS3 paradoxically results in increased JAK2V617F protein stability, increased SOCS3 phosphorylation and increased JAK2V617F phosphorylation44. These data demonstrate that regulation of JAK2 kinase activity by SOCS3 is altered in the context of the V617F substitution, and suggest the possibility that therapeutic inhibition of SOCS3 might selectively attenuate JAK2V617F, but not wild-type JAK2 signalling.

In vivo data from murine BMT experiments have provided important insights into the role of JAK2 activa-tion in the pathogenesis of MPD. James et al. noted that the expression of JAK2V617F, but not wild-type JAK2, in a murine BMT assay results in significant erythrocytosis in recipient mice 28 days after transplantation8,9, and

Figure 1 | Classification and molecular pathogenesis of the MPD. The different myeloproliferative disorders (MPD) can be classified by the predominant terminally differentiated myeloid cell involved in the disorder, and for each terminally differentiated myeloid cell there is a clinically distinct MPD. Different approaches have been used to identify the activating alleles that cause these disorders, and in all cases these alleles result in constitutive tyrosine kinase signalling. HSC, haematopoietic stem cell; JAK2, Janus kinase 2; MPL, thrombopoietin receptor; PDGFR, platelet derived growth factor receptor.

Box 1 | Myeloproliferative diseases

Myeloproliferative diseases (MPD) are proliferative syndromes that present with increased numbers of functional, mature, terminally differentiated myeloid elements. Thus, in contrast to impaired myeloid differentiation that is characteristic of myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML), patients with MPD rarely present with infectious complications related to leucopaenia, or complications related to anaemia that are observed in MDS and AML. In addition, although these MPD can be associated with bleeding tendencies (diatheses) that are observed in MDS and AML owing to thrombocytosis and platelet dysfunction, they are often associated with specific clinical sequelae that include thrombotic diatheses that are not observed in MDS or AML. However, MDS and AML are thought to be stem cell disorders, based on clonality analyses using X-inactivation-based assays in females or cytogenetic analyses that demonstrate the clonal involvement of the different haematopoietic lineages111,112.

The first MPD allele to be identified and characterized was the BCR-ABL fusion oncogene in chronic myeloid leukaemia (CML) that arises as a consequence of the 9:22 translocation2, a causative agent for this disease, and recurrent translocations led to the discovery of different fusion tyrosine kinases, including platelet derived growth factor receptor-B (PDGFRB) fusions in patients with chronic myelomonocytic leukaemia (CMML) and other MPD3. The observation that KIT is highly expressed in mast cells led to the discovery of the KITD816V allele in systemic mastocytosis (SM)5, and imatinib responses in patients with chronic eosinophilic leukaemia (CEL) and SM led to the discovery of an interstitial deletion that gives rise to the FIP1L1–PDGFRA fusion4. Most recently, investigators have identified Janus kinase 2 (JAK2) and thrombopoietin receptor mutations in polycythaemia vera (PV), essential thrombocythaemia (ET) and primary meylofibrosis (PMF) that result in the activation of JAK2 signalling8–11,53,55.

REVIEWS

NATURE REVIEWS | CANCER VOLUME 7 | SEPTEMBER 2007 | 675

Nature Reviews |

Cancer

Neutrophils

Monocytes

Myeloid

progenitor

HSC

MPD

Activati

ng

mutation

Chronic myeloid

leukaemia

Chronic

myelomonocytic

leukaemia

Primary

myelofibrosis

Mast

cell

Systemic

mastocytosis

KITD816V

FIP1L1–

PDGFRA

Red

blood cells

Polyc

ythaemia

vera

JAK2V617F

JAK2 Exo

n 12

Platelets

Essentia

l

thrombo-

cythaemia

JAK2V617F

MPLW515

L/K

JAK2V617F

MPLW515

L/K

Eosin

ophils

Chronic

eosinophilic

leukemia

FIP1L1–

PDGFRA

BCR-ABL

TEL–PDGFRB

BCR–PDGFRA

TEL–JAK2

other fusio

n TKs

Leuco

paenia

Low white

blood cell c

ount.

Erythro

cytosi

s

Increase

d red blood ce

ll count.

In vitro stu

dies dem

onstrate t

hat the e

xpressio

n of

JAK2V617F activate

s multip

le downstre

am sig

nalling

pathways8,36, in

cluding th

e Stat

family of tr

anscriptio

n

factors,

the mito

gen activate

d protein kinase (

MAPK)

signalling path

way, and the p

hosphotidylinositol 3-kinase

(PI3K)–Akt signallin

g pathway (FIG

. 2). M

ost activ

ated

tyrosine kinases th

at have been id

entified in

human

malignancies a

ctivate th

ese sa

me signallin

g casca

des,

and the r

ole and re

quirement o

f the S

tat, M

APK and

PI3K–Akt signalli

ng pathways in

JAK2V617F-m

ediated

transfo

rmatio

n of haemato

poietic ce

lls has n

ot been fully

elucid

ated. H

owever, severa

l lines o

f evidence suggest

that activ

ation of th

e Stat

family of tr

anscription fac

tors

is importa

nt in JA

K2V617F-mediate

d transfo

rmatio

n.

First, ex

pressio

n of either c

onstitutively

active S

TAT5 or

its anti-a

poptotic tar

get gene B

CL-XL in human haem

at-

opoietic p

rogenitors resu

lts in EPO-in

dependent colony

formatio

n37, a

hallmark

of human PV. Moreo

ver, STAT3

activatio

n and BCL-XL

overexpres

sion are

observed in

most PV patie

nt samples

38,39. These data

imply that S

tat

pathway act

ivation is i

mportant in

JAK2V617F-m

ediated

transfo

rmatio

n, but do not indicat

e wheth

er Stat

pathway

activatio

n is nece

ssary an

d/or suffic

ient fo

r JAK2V617F-

mediated tra

nsform

ation. M

urine b

one marr

ow trans-

plantati

on (BMT) assa

ys usin

g Stat5a;Stat5b-deficien

t

mice have b

een used to sh

ow that S

TAT5 is require

d

for haem

atopoieti

c transfo

rmatio

n by the constit

utively

active TEL–JAK2 fu

sion tyrosin

e kinase

40, and future

experiments w

ill ultim

ately dete

rmine w

hether t

he same

is true fo

r JAK2V617F. I

n additio

n, the ac

tivation of si

g-

nalling by the JA

K2V617F kinase might in

part be d

ue to

escape fr

om negative-fe

edback mech

anisms im

portant

in attenuatin

g JAK2 sig

nalling. Ja

k activity is

negatively

regulate

d by the Socs f

amily of protein

s, which

normally

bind to the Jak kinase

s and res

ult in their

degradatio

n. In

particu

lar, SOCS1 an

d SOCS3 have been

shown to bind

to JAK2 an

d inhibit JAK2 cat

alytic

activity

41,42, and over-

expressio

n of SOCS1 results i

n abrogatio

n of in vitro

and

in vivo tra

nsform

ation by TEL–JAK2 (REF. 4

3). Although

expression of SOCS1 res

ults in JA

K2 and JA

K2V617F deg-

radatio

n and inhibitio

n of kinase act

ivity, the ex

pressio

n

of SOCS3 paradoxica

lly results

in increase

d JAK2V617F

protein sta

bility, in

crease

d SOCS3 phosphorylation an

d

increased JA

K2V617F phosphorylation44. T

hese data

demonstra

te that r

egulation of JA

K2 kinase ac

tivity by

SOCS3 is alter

ed in the contex

t of the V

617F substitution,

and suggest

the possib

ility that t

herapeutic

inhibition of

SOCS3 might selec

tively att

enuate JA

K2V617F, but n

ot

wild-type JA

K2 signalli

ng.

In vivo data fro

m murin

e BMT experiments h

ave

provided importa

nt insig

hts into the ro

le of JA

K2 activa-

tion in the path

ogenesis of M

PD. James e

t al. noted

that

the expres

sion of JA

K2V617F, but not w

ild-type JA

K2, in

a murin

e BMT ass

ay results

in signific

ant eryth

rocytosis

in recip

ient m

ice 28 days a

fter tra

nsplantation8

,9, and

Figure 1 | Classi

ficatio

n and molecular p

athogenesis of th

e MPD. The diffe

rent

myeloproliferativ

e disorders (

MPD) can be classified by th

e predominant term

inally

differentia

ted myeloid cell involved in th

e disorder, a

nd for each te

rminally diffe

rentiated

myeloid cell there is a

clinically distinct M

PD. Diffe

rent approaches have been used to

identify th

e activatin

g alleles that cause th

ese disorders,

and in all cases th

ese alleles result

in constitutiv

e tyrosine kinase sig

nalling. H

SC, haematopoietic ste

m cell; JAK2, Ja

nus

kinase 2; MPL, th

rombopoietin receptor; P

DGFR, platelet deriv

ed growth factor receptor.

Box 1 | Myeloprolife

rative dise

ases

Myeloproliferativ

e diseases (M

PD) are proliferativ

e syndromes th

at present w

ith increased numbers o

f functio

nal, mature,

terminally diffe

rentiated myeloid elements.

Thus, in contra

st to im

paired myeloid diffe

rentiatio

n that is

characteristic of

myelodysplastic sy

ndromes (MDS) and acute myeloid leukaemia (A

ML), patie

nts with

MPD ra

rely present with

infectious

complications re

lated to leu

copae

nia, or complicatio

ns related to

anaemia that a

re observed in MDS and AML. In

addition,

although th

ese MPD can be asso

ciated with bleeding te

ndencies (diatheses) t

hat are observed in M

DS and AML owing to

thrombocytosis and platelet d

ysfunctio

n, they are ofte

n associated with

specific

clinical se

quelae that in

clude thrombotic

diatheses that a

re not observed in M

DS or AML. H

owever, MDS and AML are th

ought to be ste

m cell diso

rders, based on

clonality analyses u

sing X-in

activatio

n-based assays in

females o

r cytogenetic

analyses that d

emonstrate th

e clonal

involvement of th

e different h

aematopoietic lin

eages111,112.

The first M

PD allele to

be identified and characteriz

ed was the BCR-ABL fu

sion oncogene in chronic myeloid leukaemia

(CML) that a

rises a

s a consequence of th

e 9:22 translo

cation2, a causativ

e agent for th

is dise

ase, and recurre

nt

translo

cations le

d to th

e discovery of d

ifferent fu

sion ty

rosine kinases, i

ncluding platelet deriv

ed growth factor receptor-B

(PDGFRB) fusio

ns in patie

nts with

chronic myelomonocytic leukaemia (C

MML) and other MPD

3. The observation th

at KIT is

highly expressed in mast c

ells led to

the disc

overy of the KITD816V alle

le in syste

mic mastocytosis

(SM)5, and imatin

ib

responses in patie

nts with

chronic eosinophilic

leukaemia (CEL) and SM led to

the disc

overy of an interstitia

l deletion th

at

gives rise to

the FIP1L1–PDGFRA fu

sion4. M

ost recently

, investig

ators have identifi

ed Janus kinase 2 (JA

K2) and

thrombopoietin re

ceptor mutatio

ns in polycythaemia vera (PV), e

ssentia

l thrombocythaemia (ET) and prim

ary

meylofibrosis

(PMF) that re

sult in th

e activatio

n of JAK2 sig

nalling8

–11,53,55.

REVIEW

S

NATURE REVIEWS | CANCER

VOLUME 7 | SEPTEMBER 2007 | 6

75

Nature Reviews | Cancer

Neutrophils

Monocytes

Myeloid

progenitor

HSC

MPD

Activating

mutation

Chronic myeloid

leukaemia

Chronic

myelomonocytic

leukaemia

Primary

myelofibrosis

Mast

cell

Systemic

mastocytosis

KITD816V

FIP1L1–PDGFRA

Red

blood cells

Polycythaemia

vera

JAK2V617F

JAK2 Exon 12

Platelets

Essential

thrombo-

cythaemia

JAK2V617F

MPLW515L/K

JAK2V617F

MPLW515L/K

Eosinophils

Chronic

eosinophilic

leukemia

FIP1L1–PDGFRA

BCR-ABL

TEL–PDGFRB

BCR–PDGFRA

TEL–JAK2

other fusion TKs

Leucopaenia

Low white blood cell count.

Erythrocytosis

Increased red blood cell count.

In vitro studies demonstrate that the expression of

JAK2V617F activates multiple downstream signalling

pathways8,36 , including the Stat family of transcription

factors, the mitogen activated protein kinase (MAPK)

signalling pathway, and the phosphotidylinositol 3-kinase

(PI3K)–Akt signalling pathway (FIG. 2). Most activated

tyrosine kinases that have been identified in human

malignancies activate these same signalling cascades,

and the role and requirement of the Stat, MAPK and

PI3K–Akt signalling pathways in JAK2V617F-mediated

transformation of haematopoietic cells has not been fully

elucidated. However, several lines of evidence suggest

that activation of the Stat family of transcription factors

is important in JAK2V617F-mediated transformation.

First, expression of either constitutively active STAT5 or

its anti-apoptotic target gene BCL-X L in human haemat-

opoietic progenitors results in EPO-independent colony

formation37 , a hallmark of human PV. Moreover, STAT3

activation and BCL-X L overexpression are observed in

most PV patient samples38,39 . These data imply that Stat

pathway activation is important in JAK2V617F-mediated

transformation, but do not indicate whether Stat pathway

activation is necessary and/or sufficient for JAK2V617F-

mediated transformation. Murine bone marrow trans-

plantation (BMT) assays using Stat5a;Stat5b-deficient

mice have been used to show that STAT5 is required

for haematopoietic transformation by the constitutively

active TEL–JAK2 fusion tyrosine kinase40 , and future

experiments will ultimately determine whether the same

is true for JAK2V617F. In addition, the activation of sig-

nalling by the JAK2V617F kinase might in part be due to

escape from negative-feedback mechanisms important

in attenuating JAK2 signalling. Jak activity is negatively

regulated by the Socs family of proteins, which normally

bind to the Jak kinases and result in their degradation. In

particular, SOCS1 and SOCS3 have been shown to bind

to JAK2 and inhibit JAK2 catalytic activity41,42 , and over-

expression of SOCS1 results in abrogation of in vitro and

in vivo transformation by TEL–JAK2 (REF. 43). Although

expression of SOCS1 results in JAK2 and JAK2V617F deg-

radation and inhibition of kinase activity, the expression

of SOCS3 paradoxically results in increased JAK2V617F

protein stability, increased SOCS3 phosphorylation and

increased JAK2V617F phosphorylation44 . These data

demonstrate that regulation of JAK2 kinase activity by

SOCS3 is altered in the context of the V617F substitution,

and suggest the possibility that therapeutic inhibition of

SOCS3 might selectively attenuate JAK2V617F, but not

wild-type JAK2 signalling.

In vivo data from murine BMT experiments have

provided important insights into the role of JAK2 activa-

tion in the pathogenesis of MPD. James et al. noted that

the expression of JAK2V617F, but not wild-type JAK2, in

a murine BMT assay results in significant erythrocytosis

in recipient mice 28 days after transplantation8,9 , and

Figure 1 | Classification and molecular pathogenesis of the MPD. The different

myeloproliferative disorders (MPD) can be classified by the predominant terminally

differentiated myeloid cell involved in the disorder, and for each terminally differentiated

myeloid cell there is a clinically distinct MPD. Different approaches have been used to

identify the activating alleles that cause these disorders, and in all cases these alleles result

in constitutive tyrosine kinase signalling. HSC, haematopoietic stem cell; JAK2, Janus

kinase 2; MPL, thrombopoietin receptor; PDGFR, platelet derived growth factor receptor.

Box 1 | Myeloproliferative diseases

Myeloproliferative diseases (MPD) are proliferative syndromes that present with increased numbers of functional, mature,

terminally differentiated myeloid elements. Thus, in contrast to impaired myeloid differentiation that is characteristic of

myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML), patients with MPD rarely present with infectious

complications related to leucopaenia, or complications related to anaemia that are observed in MDS and AML. In addition,

although these MPD can be associated with bleeding tendencies (diatheses) that are observed in MDS and AML owing to

thrombocytosis and platelet dysfunction, they are often associated with specific clinical sequelae that include thrombotic

diatheses that are not observed in MDS or AML. However, MDS and AML are thought to be stem cell disorders, based on

clonality analyses using X-inactivation-based assays in females or cytogenetic analyses that demonstrate the clonal

involvement of the different haematopoietic lineages111,112 .

The first MPD allele to be identified and characterized was the BCR-ABL fusion oncogene in chronic myeloid leukaemia

(CML) that arises as a consequence of the 9:22 translocation2 , a causative agent for this disease, and recurrent

translocations led to the discovery of different fusion tyrosine kinases, including platelet derived growth factor receptor-B

(PDGFRB) fusions in patients with chronic myelomonocytic leukaemia (CMML) and other MPD3 . The observation that KIT is

highly expressed in mast cells led to the discovery of the KITD816V allele in systemic mastocytosis (SM)5 , and imatinib

responses in patients with chronic eosinophilic leukaemia (CEL) and SM led to the discovery of an interstitial deletion that

gives rise to the FIP1L1–PDGFRA fusion4 . Most recently, investigators have identified Janus kinase 2 (JAK2) and

thrombopoietin receptor mutations in polycythaemia vera (PV), essential thrombocythaemia (ET) and primary

meylofibrosis (PMF) that result in the activation of JAK2 signalling8–11,53,55 .

REVIEWS

NATURE REVIEWS | CANCER

VOLUME 7 | SEPTEM

BER 2007 | 675

Nature Reviews | Cancer

Neutrophils

Monocytes

Myeloid

progenitor

HSC

MPD

Activating

mutation

Chronic myeloid

leukaemia

Chronic

myelomonocytic

leukaemia

Primary

myelofibrosis

Mast

cell

Systemic

mastocytosis

KITD816V

FIP1L1–PDGFRA

Red

blood cells

Polycythaemia

vera

JAK2V617F

JAK2 Exon 12

Platelets

Essential

thrombo-

cythaemia

JAK2V617F

MPLW515L/K

JAK2V617F

MPLW515L/K

Eosinophils

Chronic

eosinophilic

leukemia

FIP1L1–PDGFRA

BCR-ABL

TEL–PDGFRB

BCR–PDGFRA

TEL–JAK2

other fusion TKs

Leucopaenia

Low white blood cell count.

Erythrocytosis

Increased red blood cell count.

In vitro studies demonstrate that the expression of

JAK2V617F activates m

ultiple downstream signalling

pathways

8,36 , including the Stat family of transcription

factors, the mitogen activated protein kinase (M

APK)

signalling pathway, and the phosphotidylinositol 3-kinase

(PI3K)–Akt signalling pathw

ay (FIG. 2). Most activated

tyrosine kinases that have been identified in human

malignancies activate these sam

e signalling cascades,

and the role and requirement of the Stat, M

APK and

PI3K–Akt signalling pathw

ays in JAK2V617F-mediated

transformation of haematopoietic cells has not been fully

elucidated. However, several lines of evidence suggest

that activation of the Stat family of transcription factors

is important in JAK2V617F-mediated transform

ation.

First, expression of either constitutively active STAT5 or

its anti-apoptotic target gene BCL-XL in human haemat-

opoietic progenitors results in EPO-independent colony

formation

37 , a hallmark of human PV. Moreover, STAT3

activation and BCL-XL overexpression are observed in

most PV patient samples38,39 . These data im

ply that Stat

pathway activation is important in JAK2V617F-mediated

transformation, but do not indicate w

hether Stat pathway

activation is necessary and/or sufficient for JAK2V617F-

mediated transform

ation. Murine bone marrow trans-

plantation (BMT) assays using Stat5a;Stat5b-deficient

mice have been used to show

that STAT5 is required

for haematopoietic transform

ation by the constitutively

active TEL–JAK2 fusion tyrosine kinase

40 , and future

experiments will ultimately determine whether the sam

e

is true for JAK2V617F. In addition, the activation of sig-

nalling by the JAK2V617F kinase might in part be due to

escape from negative-feedback m

echanisms important

in attenuating JAK2 signalling. Jak activity is negatively

regulated by the Socs family of proteins, w

hich normally

bind to the Jak kinases and result in their degradation. In

particular, SOCS1 and SOCS3 have been show

n to bind

to JAK2 and inhibit JA

K2 catalytic activity41,42 , and over-

expression of SOCS1 results in abrogation of in vitro and

in vivo transformation by TEL–JA

K2 (REF. 43). Although

expression of SOCS1 results in JAK2 and JAK2V

617F deg-

radation and inhibition of kinase activity, the expression

of SOCS3 paradoxically results in increased JA

K2V617F

protein stability, increased SOCS3 phosphorylation and

increased JAK2V617F phosphorylation

44 . These data

demonstrate that regulation of JA

K2 kinase activity by

SOCS3 is altered in the context of the V

617F substitution,

and suggest the possibility that therapeutic inhibition of

SOCS3 might selectively attenuate JA

K2V617F, but not

wild-type JAK2 signalling.

In vivo data from murine BMT experiments have

provided important insights into the role of JA

K2 activa-

tion in the pathogenesis of MPD. James et al. noted that

the expression of JAK2V617F, but not w

ild-type JAK2, in

a murine BMT assay results in significant erythrocytosis

in recipient mice 28 days after transplantation

8,9 , and

Figure 1 | Classification and m

olecular pathogenesis of the MPD. The different

myeloproliferative disorders (M

PD) can be classified by the predom

inant terminally

differentiated myeloid cell involved in the disorder, and for each term

inally differentiated

myeloid cell there is a clinically distinct M

PD. Different approaches have been used to

identify the activating alleles that cause these disorders, and in all cases these alleles result

in constitutive tyrosine kinase signalling. HSC, haematopoietic stem

cell; JAK2, Janus

kinase 2; MPL, thrombopoietin receptor; PD

GFR, platelet derived grow

th factor receptor.

Box 1 | Myeloproliferative diseases

Myeloproliferative diseases (M

PD) are proliferative syndrom

es that present with increased num

bers of functional, mature,

terminally differentiated m

yeloid elements. Thus, in contrast to im

paired myeloid differentiation that is characteristic of

myelodysplastic syndrom

es (MDS) and acute m

yeloid leukaemia (AML), patients with MPD rarely present w

ith infectious

complications related to leucopaenia, or com

plications related to anaemia that are observed in M

DS and AML. In addition,

although these MPD can be associated w

ith bleeding tendencies (diatheses) that are observed in MDS and AML owing to

thrombocytosis and platelet dysfunction, they are often associated w

ith specific clinical sequelae that include thrombotic

diatheses that are not observed in MDS or AML. However, MDS and AML are thought to be stem

cell disorders, based on

clonality analyses using X-inactivation-based assays in females or cytogenetic analyses that dem

onstrate the clonal

involvement of the different haem

atopoietic lineages111,112 .

The first MPD allele to be identified and characterized w

as the BCR-ABL fusion oncogene in chronic m

yeloid leukaemia

(CML) that arises as a consequence of the 9:22 translocation

2 , a causative agent for this disease, and recurrent

translocations led to the discovery of different fusion tyrosine kinases, including platelet derived growth factor receptor-B

(PDGFRB) fusions in patients w

ith chronic myelomonocytic leukaem

ia (CMML) and other M

PD3 . The observation that KIT is

highly expressed in mast cells led to the discovery of the KITD

816V allele in systemic mastocytosis (SM

)5 , and imatinib

responses in patients with chronic eosinophilic leukaem

ia (CEL) and SM led to the discovery of an interstitial deletion that

gives rise to the FIP1L1–PDGFRA fusion4 . M

ost recently, investigators have identified Janus kinase 2 (JAK2) and

thrombopoietin receptor m

utations in polycythaemia vera (PV), essential throm

bocythaemia (ET) and prim

ary

meylofibrosis (PM

F) that result in the activation of JAK2 signalling8–11,53,55 .

REVIEWS

NATURE REVIEWS | CANCER

VOLUME 7 | SEPTEMBER 2007 | 675

Myeloproliferativeneoplasms:JAK2gain of functionmutationas common oncogene

Pikman etal,PLoS Med2006;Klampfl etal/Nangalia etal,NEJM2013

modifiedfromMeyerSCetal,Clin CancerRes2014Rampal Retal,Blood2014

Myeloproliferativeneoplasms arealldriven by increased JAK2signaling activity

MPL